1. Field of the Invention
This invention relates generally to tube-type audio amplifiers, and more specifically to an output tube bias selection apparatus enabling tone alteration of the output signal.
2. Description of the Prior Art
Tube type audio amplifiers, especially tube type guitar amplifiers, have had their output tube stages configured as class A (single ended or push pull, with no grid current); class AB1 (push pull with no grid current); and class AB2 (with some grid current allowed during some portion of the signal drive cycle). In those push pull type output stages, pairs or multiple pairs of output tubes are employed to achieve relatively large output wattages. In all of these cases however, some means of setting up a quiescent bias level of current flowing within the output tubes is necessary to allow the tubes to operate over some useful portion of their characteristic transfer curves. This quiescent current level, when properly selected, also reduces or eliminates the crossover or notch distortion present in pure class B operation (where no quiescent current is employed).
Two basic methods of achieving this quiescent current goal have been developed. These are called methods of bias and they are: 1) fixed bias, and 2) self or cathode bias. In fixed bias, a separate C- power supply or battery is used to D.C. bias the control grid of an output tube (with respect to that tubes's cathode) at such a point that a suitable plate current flows in that tube. If we assume a constant A.C. signal drive level to the control grids of the push pull output tube, then by adjusting the voltage of this C- bias supply, that quiescent plate current may be adjusted to achieve the typical class A or class AB operation so desirable in audio output stages. In this fixed bias system, the cathodes of the output tubes are usually connected to ground (low side of the B+ supply) through a very small cathode resistor (typically less than ten ohms), or connected to ground directly. In some rare cases, designers have used fairly large cathode resistors to achieve a combination of fixed and self bias, but that is not pertinent to the instant invention.
In self or cathode bias operation, the quiescent bias current through the output tubes is achieved by holding the control grid of an output tube at D.C. ground potential and utilizing a suitable resistor in the cathode circuit of that tube connected to ground (B-). With a high D.C. potential applied to the plate (and screen grid) of that output tube, a current begins to flow through that cathode resistor (and through the output tube). This flowing current causes a voltage to appear across that cathode resistor, as predicted by Ohm's Laws. This voltage is effectively applied between the control grid and cathode of that tube. This achieves what the C- battery (or C- direct current power supply) accomplished in the fixed bias scheme described above. Since the current through the tube causes a voltage to build up across the cathode resistor and hence control the quiescent current through that tube itself, this bias method is called self or cathode bias.
In the past, tube type guitar audio amplifiers have employed either fixed bias or self bias as mentioned above. Self bias was very common in 1950's genre of tube type guitar amplifiers, while fixed bias became more common in 1960's style amplifiers, with some crossover between the two types. In one obscure example of an amplifier which used both, fixed bias was the dominant and preferred bias form, and only when the separate C- bias supply failed, did the design switch over automatically to the self bias mode. However, the type of bias selection was not user controllable, and employed a relay to do the switching between the bias methods automatically, since self bias was only used as a fail-safe measure in case of component failure. It was not used, as in the current invention, as a tonal deciding feature.